MRI is a diagnostic test that uses a magnetic field and radio waves to show structural and chemical changes in tissues. This technique is more sensitive than X-rays in detecting some changes that occur in diseases of the brain. For the procedure, the patient lies on a table that slides into a metal cylinder (the scanner). The confined space may produce anxiety in some patients, and patients can talk to the technician at all times during the procedure. Earplugs are provided to muffle loud knocking and pulsing noises that occur while the scanner is taking pictures. During the study, the contrast material gadolinium may be injected into an arm vein. Gadolinium "brightens" the pictures, producing better images of brain blood flow.